There has been a lot of interest in flexible, lightweight, and high-power energy devices for wearable smart cloth and miniaturized electronic applications. To meet the demands for such applications, recent research has focused on dimension conversion of energy devices from three- or two-dimensional (3D, 2D) types to one-dimensional (1D) fibrous structure. Such a trend is well demonstrated in energy generation or conversion fields, for example, fiber photovoltaic cells, fiber piezoelectric generators, fiber thermoelectric generators, and fiber biofuel cells.
As for supercapacitors, one of the next-generation energy storage media for a high level of electrical power and long lifetime, nanowire-microfiber hybrid-structure supercapacitors, a pen ink decorated metal wire supercapacitor, and a self-powered system integrated supercapacitor have recently been reported. Nevertheless, such fiber supercapacitors still suffer from complicated fabrication methods and complex structures and have low flexibility. The fiber supercapacitors are only slightly bendable, which limits their application to large-size devices and wearable and portable electronics where flexibility is needed.
Meanwhile, realizing high electrochemical performance of supercapacitors is another important issue. Especially for supercapacitors based on manganese oxide (MnO2), a promising transition metal oxide as a pseudo-capacitive material with high theoretical capacitance, low cost, natural abundance, and environmental friendliness, overcoming the poor electrical conductivity (10−5-10−6 S/cm) of the MnO2 still remains an unavoidable challenge to be addressed for optimization of its charge storage performance. Accordingly, several research groups have introduced some structural strategies for electrode design in order to enhance the electrical conductivity and facilitate the full utilization of MnO2 by incorporating metal oxide or metal-based nanostructures as an effective electron pathway. For example, a variety of nanowires, such as SnO2, ZnO, ZnSnO4, Co3O4, and WO3, have been grown on the surface of current collectors and nanoscopic MnO2 deposited on them to fabricate core-shell-structured hybrid electrodes [(a) J. Yan et al. ACS Nano 2010, 4, 4247; (b) J. Bae et al. Angew. Chem., Int. Ed. 2011, 50, 1683; (c) L. Bao et al. Nano Lett. 2011, 11, 1215; (d) J. Liu et al. Adv Mater. 2011, 23, 2076; (e) X. Lu et al. Adv Mater. 2012, 24, 938]. In addition, nanotube arrays of Mn have been synthesized and the tube surface oxidized to make a manganese dioxide/manganese/manganese dioxide sandwich-structured electrode [Q. Li et al. Nano Lett. 2012, 12, 3803]. High electrolyte surface area and a fast charge storage process have been effectively achieved by such uniquely designed architectures, resulting in high specific capacitance and rate capability. However, these electrodes require complex multistep fabrication processes for growing nanostructures and they can be sensitive to mechanical deformation such as folding or twisting, thus being unsuitable for real applications.
Korean Patent No. 1,126,784 discloses a hybrid supercapacitor having a non-woven fabric structure in which manganese oxide is deposited on a PAN-based carbon nanofiber produced by electrospinning. The hybrid supercapacitor exhibits the functions of both a high-capacity pseudocapacitor and a double layer capacitor, achieving high energy and power densities. However, due to its low tensile strain or flexibility, the hybrid supercapacitor is difficult to utilize in electronics subjected to high strain rates and wearable and portable electronic textiles.